In the measurement of infrared (IR) spectra of fluid samples, the fluid sample and the sample cell which contains the sample in the spectrophotometer are heated by the IR radiation. The heating is unavoidable since infrared radiation is essentially heat radiation and as absorption of the radiation occurs, the sample experiences a heating effect. However, it is often desirable to remove heat from the sample during the IR spectrophotometry measurement. For example, liquid volatile solvents and samples which are heated by the IR radiation can vaporize and form bubbles inside a conventional liquid cell, particularly in a continuous-wave instrument. These bubbles cause spiking on the spectrum and usually degrade the spectrum severely before the measurement is complete. In general, better quality spectra are obtained when the sample cell is cooled in some manner.
In other instances, it is often desirable to protect thermally sensitive compounds from excessive heating and resulting degradation during the measurement of their spectra. This is particularly true for natural products and pharmaceuticals wherein infrared spectrophotometry is often used in the study of molecular association. In these studies, the sample cell must be carefully thermostated and the temperature throughout the cell must be uniform in order to minimize experimental error.
The Andrychuk U.S. Pat. No. 2,917,629 discloses an apparatus for infrared analysis of a body of chlorine in a refrigerated cell. The refrigerated cell comprises an open, double walled container in which a refrigerant is circulated between the walls. The device is specific to the measurement of the infrared spectra of liquid chlorine, and lacks application as a multipurpose sample cell. The Johnson U.S. Pat. No. 4,982,089 discloses a method of obtaining the spectra of a product wherein the product is first passed through a gas chromatograph and separated into components, and the resulting components are passed through a chilled region to a spectrometer. The cooling apparatus and method disclosed in these references are both cumbersome and ineffective in avoiding the above-noted heating problems.
Other conventional means for cooling a sample cell for infrared spectrophotometry include commercial thermostats including a cryogenic Dewar flask containing, for example, liquid nitrogen. The thermostat is generally mounted above the sample cell and counterbalance heaters are mounted in the sides of the cell. However, there are several disadvantages associated with these devices. First, the devices are relatively expensive and the cryogenic fluid consumption is high, whereby the small Dewar flask must be frequently refilled. Additionally, the technique of heat balancing with the strip heaters to achieve the desired temperature inevitably leads to undesirable temperature gradients within the cell and sample compartment. The temperature gradients in turn produce density gradients in the liquid samples, thereby preventing the accurate measurement of spectra. Further, there is usually a large and unavoidable heat leak through the windows of the cell, particularly the window closest to the irradiation source, commonly referred to as the entry surface or primary optical surface of the cell. Because the surface area to liquid volume ratio is very high for a sample cell for infrared spectrophotometry, such a heat leak can be very detrimental to the accurate measurement of sample spectra.
Accordingly, a need exists for an improved sample cell for infrared spectrophotometry which accommodates heat removal and temperature control of the sample contained in the cell.